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<title>LD: Basic Script Concepts</title>

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<a name="Basic-Script-Concepts"></a>
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Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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<a name="Basic-Linker-Script-Concepts"></a>
<h3 class="section">3.1 Basic Linker Script Concepts</h3>
<a name="index-linker-script-concepts"></a>
<p>We need to define some basic concepts and vocabulary in order to
describe the linker script language.
</p>
<p>The linker combines input files into a single output file.  The output
file and each input file are in a special data format known as an
<em>object file format</em>.  Each file is called an <em>object file</em>.
The output file is often called an <em>executable</em>, but for our
purposes we will also call it an object file.  Each object file has,
among other things, a list of <em>sections</em>.  We sometimes refer to a
section in an input file as an <em>input section</em>; similarly, a section
in the output file is an <em>output section</em>.
</p>
<p>Each section in an object file has a name and a size.  Most sections
also have an associated block of data, known as the <em>section
contents</em>.  A section may be marked as <em>loadable</em>, which means that
the contents should be loaded into memory when the output file is run.
A section with no contents may be <em>allocatable</em>, which means that an
area in memory should be set aside, but nothing in particular should be
loaded there (in some cases this memory must be zeroed out).  A section
which is neither loadable nor allocatable typically contains some sort
of debugging information.
</p>
<p>Every loadable or allocatable output section has two addresses.  The
first is the <em>VMA</em>, or virtual memory address.  This is the address
the section will have when the output file is run.  The second is the
<em>LMA</em>, or load memory address.  This is the address at which the
section will be loaded.  In most cases the two addresses will be the
same.  An example of when they might be different is when a data section
is loaded into ROM, and then copied into RAM when the program starts up
(this technique is often used to initialize global variables in a ROM
based system).  In this case the ROM address would be the LMA, and the
RAM address would be the VMA.
</p>
<p>You can see the sections in an object file by using the <code>objdump</code>
program with the &lsquo;<samp>-h</samp>&rsquo; option.
</p>
<p>Every object file also has a list of <em>symbols</em>, known as the
<em>symbol table</em>.  A symbol may be defined or undefined.  Each symbol
has a name, and each defined symbol has an address, among other
information.  If you compile a C or C++ program into an object file, you
will get a defined symbol for every defined function and global or
static variable.  Every undefined function or global variable which is
referenced in the input file will become an undefined symbol.
</p>
<p>You can see the symbols in an object file by using the <code>nm</code>
program, or by using the <code>objdump</code> program with the &lsquo;<samp>-t</samp>&rsquo;
option.
</p>
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Next: <a href="Script-Format.html#Script-Format" accesskey="n" rel="next">Script Format</a>, Up: <a href="Scripts.html#Scripts" accesskey="u" rel="up">Scripts</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
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